1. Field of the Invention
The present invention relates to a method and apparatus for managing hydrocephalus in a patient. More particularly, the invention relates to a method and apparatus for adjusting, controlling or programming the drainage rate of cerebrospinal fluid (CSF) in a hydrocephalus patient. Even more particularly, the invention relates to a shunt system for varying the opening pressure and/or diameter of the shunt and thus controlling the rate of fluid flow (drainage) of cerebrospinal fluid from a ventricular cavity.
2. Description of Related Art
Hydrocephalus is a condition afflicting patients who are unable to regulate cerebrospinal fluid flow through their body's own natural pathways. Cerebrospinal fluid (CSF) is normally produced by the choroid plexus of the brain and carries essential nutrients, hormones, and other cellular components to various portions of the brain as the fluid circulates through the ventricular system. Moreover, the CSF also helps absorb shock and cushions the brain as the fluid diffuses over the brain and spinal cord. Cerebrospinal fluid that is not recirculated eventually drains into the sagittal sinus where it is naturally absorbed by the body's venous system. In a patient suffering from hydrocephalus, the CSF absorption rate fails to keep up with the production rate, either because of an obstruction along the natural CSF pathway or due to diseased choroid plexus which increases CSF formation. The unabsorbed or excess CSF accumulates in the ventricles of the patient's brain, leading to an increase in intracranial pressure. If left untreated, the increased intracranial pressure can lead to serious medical conditions such as compression of the brain tissue and impaired blood flow to the brain, with such potential consequences as coma and/or death.
The conventional treatment for hydrocephalus patients has involved draining the excess fluid away from the ventricles and rerouting the excess CSF to another area of the patient's body, such as the peritoneum (abdomen) or vascular system. An implantable drainage system, commonly referred to as a shunt such as that disclosed in U.S. Pat. No. 4,595,390, is often used to transfer fluid so as to restore the balance between the production and absorption of CSF in the patient.
The shunt has several basic components. A first portion is the called the proximal, head or ventricular catheter implanted into the ventricular cavity of the patient's brain. The proximal catheter, in turn, is connected to the valve and reservoir. The valve controls how much fluid is drained from the brain, it is then stored in the reservoir until it is released to pass via the distal, peritoneum or drainage catheter. Once again, the distal or drainage catheter leads the excess CSF to drain to a predetermined absorption site (e.g., the abdomen (peritoneum)) of the patient's body where it will be absorbed.
A shunt performs two basic operations or functions. It allows the fluid to flow only in one direction when the intracranial pressure has exceeded some predetermined value (usually referred to as the “opening pressure” for the shunt). This system regulates the amount of the CSF in the body so that the correct amount of fluid (neither too much, nor too little) is released from the brain.
To regulate the flow of cerebrospinal fluid between the proximal and distal ends of the shunt system, the main body of the shunt usually includes a pump or a control valve. Generally, shunt systems include a valve mechanism that operates by permitting fluid flow only once the fluid pressure reaches a certain threshold level. That is, fluid enters the valve only when the fluid pressure overcomes the valve mechanism's resistance to open. Some valve mechanisms permit the non-invasive adjustment, or programming, of the opening pressure level at which fluid flow commences.
Shunts having valve mechanisms that continuously drain CSF at a fixed rate are well known, as are shunts with valves that control and/or adjust the opening pressure and/or drainage rate of the patient's CSF.
US Patent Publication No. 2005/0055009, assigned to Codman & Shurtleff, Inc., discloses an adjustable drainage system for regulating cerebrospinal fluid flow in a hydrocephalus patient where the drainage rate is adjusted in response to the ventricular volume variations in the patient. The adjustable resistance valve 40 includes a multi lumen catheter 48 having a plurality of different resistances and a selection mechanism 44 by way of a rotatable disc 46 with a single aperture 46a. During adjustment, disc 46 is rotated via an actuator 42 so that the aperture 46 aligns with the lumen having the desired resistance. Selection of the desired resistance for the adjustable resistance valve 40 is achieved by rotation of the actuator 42. This shunt configuration is disadvantageous in that it is limited in the range of resistances offered by the number of different lumens provided. Only step or incremental changes can be made to regulate the drainage as defined by the different resistances employed. Fewer pressure increments produces greater variability. Furthermore, as acknowledged in the publication itself, the resistors are prone to being clogged with particulate matter such as blood cells. Lastly, following implantation, the selection of a particular resistance in the adjustable resistance valve is accomplished using a magnetic tool that influences complementary magnets associated with the implant. As is well known, magnets are subject to unwanted external magnetic influences.
First described by F. F. Reuss in 1809, electro-osmosis is the motion of polar liquid through a membrane or other porous structure under the influence of an applied electric field. U.S. Pat. No. 6,019,882 discloses an electrokinetic high pressure hydraulic system in which an electric potential provides a means for imparting net power to the fluid and by this means to transmit and use this net power to perform work (apply force) on some system. Two specific embodiments are disclosed. A first valve embodiment is shown in FIGS. 1A & 1B. Valve 100 includes a T-shape flow system in which a microchannel 110 contains a porous dielectric 120, extending past outlet 145 about 1-2 channel diameters. Fluid inlet 140 and outlet 145 in communication with microchannel 110 provide for the flow of a fluid (liquid or gas) 150 therethrough. In order to close communication between fluid inlet 140 and outlet 145 an electric potential is applied by a power supply to spaced electrodes 130 to provide the electro-osmotic force required to move electrolyte 115 to close fluid outlet 145, and prevent fluid 150 from flowing through outlet 145. Valve 100 can be opened by simply shutting off the electric potential applied to spaced electrodes 130. In addition, valve 100 can be caused to operate in the opposite direction by simply reversing the sign of the electric potential applied to spaced electrodes 130. In accordance with this embodiment, the electrolyte used in the system (to control flow) mixes with the fluid the system is controlling. Such a mixing of the electrolyte with the CSF is impermissible in an implantable shunt system since the mixed fluid (electrolyte and CSF) would be drained into another location within the body; and introducing such an electrolyte into the body would be detrimental from a biological perspective. Even if such mixing was acceptable, the present inventive system is implanted and thus does not offer ease of access to recharge the pump with new electrolyte to replace the amount that has drained due to operation.
An alternative valve configuration is shown in FIG. 5 of U.S. Pat. No. 6,019,882. In this second embodiment, a cavity is divided into two chambers 20, 25 separated by a fluid tight flexible member 30. A fluid stream enters chamber 25 through fluid inlet line 40 and exits through fluid outlet line 35. The flow of the fluid stream is controlled by applying hydraulic pressure generated by electro-osmotic pump 170 through inlet line 45 to the fluid contained in chamber 20 and, in turn, on flexible member 30 causing it to deform and thereby close off fluid inlet line 40 and stop fluid flow. To open valve 5 the polarity of the electric potential applied to spaced electrodes 130 is reversed. The flow of fluid through fluid inlet line 40 is controllable between one of only two possible states. Valve 5 is either open thereby permitting full flow through the fluid inlet line or closed off completely. This alternative patented embodiment therefore does not permit what is hereinafter referred to as “fine titration” that allows for additional adjustments in fluid flow aside from only the two flow states of full fluid flow (OPEN state) and fluid flow cut off completely (CLOSED state). That is, fine titration is not limited to only two flow states of full fluid flow and fluid flow cut off completely, but is capable of a range of fine adjustments in between.
Neither patented embodiment discloses an electrokinetic pump without intermixing between the pump electrolyte and fluid being finely varied or titrated. Furthermore, the patented system is directed to high-pressure industrial or analytical systems for generating a pressure greater than 2500 psi (corresponding to 172,368.9 mbar or 129,287.3 mmHg) with gross open-close control. In contrast, an implantable programmable shunt system operates at a much lower and limited pressure range with fine titration. Normal intracranial pressure has a baseline in a range of approximately 13 mbar through approximately 20 mbar (corresponding to a range of approximately 10 mmHg through approximately 15 mmHg), with amplitude variations in a range of approximately 4 mbar through approximately 7 mbar (corresponding to a range of approximately 3 mmHg to approximately 5 mmHg). While maximum pathologic values may range from approximately −45 mbar (corresponding to approximately −33 mmHg) to approximately 130 mbar (corresponding to approximately 100 mmHg). Pressures beyond this maximum range may be lethal or at the very least detrimentally affect the patient. Specifically, pressures beyond 500 mbar would be physiologically irrelevant. Use of the present inventive electrokinetic actuator in an implantable shunt valve system would also require the drainage of CSF in relatively small units of ml/day.
It is therefore desirable to develop an electrokinetic actuator capable of adjusting, controlling or programming the fine titration of fluid flow through a valve mechanism, in addition to full fluid flow and/or complete cut off of fluid flow, without any intermixing between the pump electrolyte and fluid being titrated while functioning in the physiological pressure ranges of interest.